1. Field of the Disclosure
The present disclosure relates generally to process control systems and, more particularly, to emergency shutdown systems used in process control systems.
2. Brief Description of Related Technology
Industrial process control systems generally incorporate an emergency shutdown system to transition a shutdown valve to a safe state in the event of a device failure, power failure or other emergency. The emergency shutdown system is often configured such that the safe state is a default position or state for the shutdown valve. In this way, the safe state is reached in spite of an emergency that cuts power to the shutdown system or one of its components. For example, the shutdown valve may travel to the fully open or closed position to reach the safe state, while under normal operating conditions it remains fully closed or open (i.e., the normal state).
A basic emergency shutdown system includes a shutdown controller and a solenoid valve to actuate the shutdown valve. The solenoid valve provides the only mechanism to transition the shutdown valve to the safe state, and human observation is the only means to confirm the position of the shutdown valve during transitions between the normal and safe states, as well as during test procedures.
An improved emergency shutdown system includes a shutdown controller, a solenoid valve, and a valve positioner, such as a Digital Valve Controller (DVC). The solenoid valve provides one mechanism to transition the shutdown valve to the safe state, while the DVC monitors and confirms the position of the shutdown valve during transitions between the normal and safe states, as well as during test procedures. The DVC also provides a second, redundant mechanism to transition the shutdown valve to the safe state. Reliability of the shutdown system is improved relative to the basic, single solenoid implementation due to the capabilities of (i) conducting and monitoring tests of the shutdown valve, and (ii) redundantly operating the valve.
In a system with both a solenoid and a DVC, the shutdown controller typically controls the solenoid valve and the DVC via two, separate current loops. The redundancy of two, separate current loops enables the shutdown controller (and the rest of the process control system) to enable a transition to the safe state by cutting power in the first loop to the solenoid valve, while still digitally communicating with the DVC via the other loop. Unfortunately, the capability of monitoring that transition comes at the expense and complexity of installing, maintaining and operating the separate, additional loop for the DVC.